Large head forgings are currently facing issues such as uneven wall thickness, local thinning, and wrinkles. To address these problems, this paper presents the optimization of the head forging process.
The study uses Deform-3D numerical simulation analysis to compare the head punching process. By optimizing the forming slabs and punching aids, local thinning and wrinkling of the head are prevented and the optimized head forming process meets the requirements.
The head is a crucial component in equipment such as chemical and nuclear power plants. As performance requirements for these systems increase, the head must be able to withstand high temperatures and pressures for extended periods of time. Integral forging provides the head with higher strength and better resistance to high temperature and high pressure hydrogen, making it a promising solution for a wide range of applications.
The integrity of nuclear power heads is crucial to the safety and lifespan of nuclear reactors. These heads are subjected to high temperatures and pressures during operation and the materials used in nuclear reactor pressure vessels must meet strict requirements, including:
- Appropriate strength and toughness at both room temperature and working temperature, with the lowest possible brittle transition temperature.
- Good weldability and workability, both hot and cold.
- Maximum tissue stability at the working temperature.
- Sufficient hardenability and uniformity of thick cross-section texture.
Two common types of heads are ellipsoidal and spherical. This article focuses on spherical head punch forming and optimizes the head forming aids through numerical simulation to ensure uniform wall thickness of the spherical head after punching.
Brief Introduction of Spherical Head Forging Process
Cutting the bottom, feed head and tongs hold with ingot→ Upsetting and compression→ Drawing a long cutting→ Upsetting and compression→ Upsetting process→ Heat treatment after forging slab→ Slab Roughing→ Slab Inspection→ Slab bending.
Deform model establishment
The three-dimensional solid model is created using the 3D modeling software UG. This model represents the spherical head forming upper mold and its finishing, as illustrated in Figures 1 and 2.
Figure 1 Spherical head forming upper die
Figure 2 Finished drawing of spherical head
During the formation process of the spherical head, the forming upper die is connected to the movable crossbeam of the hydraulic press. The forming lower die is placed on four corner posts, and the punching stroke is set to 1300mm. After the punching process is completed, the movable crossbeam of the hydraulic press is lifted, and the finished head is lifted using a crane.
It is recommended to use SA508-3 as the material model. This type of steel has exceptional process stability, weldability, and high strength.
Please note that this material data is not included in the Deform material library. You will need to obtain the actual stress-strain curve of the material through material property experiments. This information is shown in Figure 3.
Figure 3 SA508-3 material stress-strain curve
During the slab forming process, the forming slab is referred to as a deformed body and the forming die as a rigid body. The friction between the forming slab and the mold is a complex physical phenomenon that depends on various factors of the contact surface, such as relative hardness, surface roughness, temperature, normal stress, and relative sliding speed between contact surfaces, which also change during the deformation process.
There are two types of friction in the Deform process: shear friction and Coulomb friction. This article focuses on shear friction. The direct friction coefficient between the deformed body and the upper mold is defined as μ = 0.4, while the friction coefficient between the deformed body and the lower mold is defined as μ = 0.3. The punching temperature was set to 1000°C.
Numerical Simulation Analysis of Spherical Head Forming
According to the dimensions of the final illustration, a slab illustration is devised. Numerical simulations are employed in a sequence to enhance the slab and mold.
The dimensions of the slab are confirmed through simulation outcomes prior to forming, to establish the forming slab. As depicted in Figure 4.
Figure 4 Forming slab size
After the punching process (with a stroke of 1300mm), the comparison between the spherical head forging and the finished product is presented in Figure 5. The numerical simulation of the spherical head forgings produced using this method satisfies the requirements for a finished product.
The remaining margin at the bottom of the spherical head is approximately 10mm on one side, and the open end of the spherical head measures about 25mm on one side.
As depicted in the figure, the spherical head forging meets the necessary dimensions for a finished product.
Figure 5 Comparison of spherical head forgings and refined drawings
The equivalent stress after forming is displayed in Figure 6, where the maximum deformation at the open end of the spherical head results in a concentrated stress of approximately 40 MPa. On the other hand, the minimum deformation at the bottom of the spherical head leads to the minimum forming stress.
Figure 7 shows the equivalent strain after forming, with an iso effect ranging from 0.02 to 0.2 mm/mm. Figure 8 displays the forming force, with the maximum forming force reaching approximately 2600 t.
The numerical simulation results for spherical head punching indicate that the bottom thinning is particularly severe. To mitigate this issue, you need to adjust the forming angle of the lower die, optimize the arc at the bottom of the upper die, and ultimately ensure uniform head margin after punching.
Figure 6 Equivalent stress diagram of spherical head after punching
Fig. 7 Equivalent strain diagram of spherical head after punching
Figure 8 Forming force of spherical head punch
Optimization and Numerical Simulation of Spherical Head Punching Die
First, analyze the impact of the punching die angle on the head slab forming process, then adjust the forming die angle and compare the forming force.
After simulating and comparing, there is no significant difference between the forgings and the finishing allowances.
However, due to the smaller head diameter and larger length of the straight section of the regulator, it has been observed that as the punching die forming angle increases, the punching force decreases.
A better guiding effect during punching of the slab reduces the likelihood of wrinkling.
With the forming temperature set at 1000°C and the slab shape unchanged, the comparison of forming forces is illustrated in Figure 9.
Figure 9 Effect of die angle on forming force
The larger the mold angle, the lower the forming force required.
When the lower die angle is set at 16°, the forming force required is 2600 t. At an angle of 21°, the force required drops to 2400 t, and at 35°, it decreases further to 2000 t.
It is evident from these comparisons that a larger angle results in a lower forming force.
Therefore, based on the analysis of forming forces, the final angle for the spherical head punching die was determined to be 35°.
To improve the forming slab, the thickness of the bottom of the spherical head was increased and the bottom was extended by 25 mm. This was done to ensure a uniform wall thickness of the spherical head slab after punching and to provide additional margin for the bottom of the spherical head. The modified slab is illustrated in Figure 10.
Figure 10 Optimized forming slab
After optimizing the spherical head slab and the punching tools, the results of the spherical head punching are depicted in Figure 11.
Figure 11 Spherical head punching
The maximum equivalent stress at the open end of the spherical head is approximately 40 MPa.
A comparison between the spherical head forging post-punching and post-finishing can be seen in Figure 12.
Figure 12 Comparison of spherical head forging and finishing
The spherical head forgings display uniform margins across all parts, with the inner wall of the forgings in good alignment with the punch and consistent margins.
The margin on the bottom of the spherical head measures approximately 20 mm, while the margins on both sides of the open end are approximately 20 mm, which satisfies the requirements. The completed spherical head forging is presented in Figure 13.
Figure 13 Spherical head forging
(1) When designing the spherical head to form a slab, it is important to increase the wall thickness at the critical point. The design angle of the punch die should be between 30° and 40° to prevent local thinning of the head due to the increased forming force. Numerical simulations are performed to continuously optimize the slab and forming process to ensure that the head is punched and formed in one step, preserving the entire forging process for a solid structural foundation for subsequent heat treatment.
(2) To prevent damage to the spherical head during punching, the gap between the punch and die should be adjusted, and the fillet of the die should be optimized. It is crucial to control the speed and stroke of the hydraulic press. The press should use low pressure when starting the punching process to avoid excessive speed.
(3) Once the punch contacts the head slab, the forging endpoint is set when the punching stroke reaches H = 600-700mm, allowing for full release of internal stress during the punching process. If wrinkling or folding occurs during punching, the process should be immediately stopped. After reheating the slab in the furnace, punching should be performed again to prevent the head slab from becoming stuck on the lower die, avoiding thinning at the bottom of the head.
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